A major genetic locus controlling natural Plasmodium falciparum infection is shared by East and West African Anopheles gambiae

Abstract

Background: Genetic linkage mapping identified a region of chromosome 2L in the Anopheles gambiae genome that exerts major control over natural infection by Plasmodium falciparum. This 2L Plasmodium-resistance interval was mapped in mosquitoes from a natural population in Mali, West Africa, and controls the numbers of P. falciparum oocysts that develop on the vector midgut. An important question is whether genetic variation with respect to Plasmodium-resistance exists across Africa, and if so whether the same or multiple geographically distinct resistance mechanisms are responsible for the trait.

Methods: To identify P falciparum resistance loci in pedigrees generated and infected in Kenya, East Africa, 28 microsatellite loci were typed across the mosquito genome. Genetic linkage mapping was used to detect significant linkage between genotype and numbers of midgut oocysts surviving to 7-8 days post-infection.

Results: A major malaria-control locus was identified on chromosome 2L in East African mosquitoes, in the same apparent position originally identified from the West African population. Presence of this resistance locus explains 75% of parasite free mosquitoes. The Kenyan resistance locus is named EA_Pfin1 (East Africa_ Plasmodium falciparum Infection Intensity).

Conclusion: Detection of a malaria-control locus at the same chromosomal location in both East and West African mosquitoes indicates that, to the level of genetic resolution of the analysis, the same mechanism of Plasmodium-resistance, or a mechanism controlled by the same genomic region, is found across Africa, and thus probably operates in A. gambiae throughout its entire range.

Figure 1

Physical positions of microsatellite markers used in genetic mapping. The three chromosomes are indicated as horizontal lines (from top: X, 3, 2), and positions of marker loci are vertical lines labeled with the marker name. Note the high density of markers on chromosome 2L (boxed section expanded in bottom horizontal line, labeled 2L box), which were used to map at greater density once an initial linkage signal was detected. Dense markers were necessary so that the most informative markers (i.e., those with the greatest numbers of alleles segregating) could be used in the analysis. The five clustered marker loci giving significant linkage (linkage data shown in Table 1) are depicted in bold red type and are starred. For comparison, the extent of the Plasmodium-resistance island mapped in West African pedigrees [4] is indicated by the filled bar above chromosome 2L (labeled PRI). Chromosome 2 is shown in the 2La inverted (2La/a) conformation.

Figure 2

Pedigree criteria for genetic analysis. Pedigrees were chosen for genome-wide genotyping and linkage analysis based on unbiased criteria of infection prevalence (the percent of mosquitoes with at least one midgut oocyst) and pedigree size. These criteria, previously applied in the West African pedigree study [4], prioritize the full analysis of the potentially most informative pedigrees. Pedigrees with ≥20 mosquitoes were analysed (actual range 25–36) and ≥30% infection prevalence (actual range, 31–64%). Only 6 of 28 pedigrees met this criterion, shown in the upper right quadrant. Pedigrees falling outside these boundaries are unlikely to yield a detectable genetic signal, even if segregating strong alleles for resistance/susceptibility. The horizontal line inside the graph indicates 30% prevalence and the vertical line indicates a pedigree size of 20. The red x indicates the pedigree that gave a significant linkage signal.

Figure 3

Mapping of the chromosomal region containing EA_Pfin1. Nominal p values are plotted as a function of physical position. For reference, a value of 1.3 on the transformed y-axis corresponds to a p value of 0.05, shown as a horizontal line. The order of markers uses the inverted (2La/a) conformation of the 2L chromosome. Only the most informative markers (≥2 alleles and ≥3 genotypes) are shown. Only markers H796, H17686896, and H25C are fully informative, that is, segregate 4 alleles so that a unique microsatellite allele marks each parental chromosome that entered the pedigree. The dip in the center of the plot is likely due to a lack of marker informativeness.

Figure 4

Oocyst distribution and allele frequency at microsatellite marker H796. Four alleles are segregating at H796, but only two are informative for infection outcome. Allele 82 (blue) is linked to low oocyst number (average oocyst number 0.08) and allele 84 (red) is linked to high oocyst number (average oocyst number 2.9). Similar histograms can be drawn for the other loci yielding significant linkage with infection outcome.